1. Field of the Invention
The present invention relates to a color image formation apparatus, particularly to a color image formation apparatus according to an electrophotographic method by which a high quality of image excellent in gray balance can be stably formed.
2. Description of the Related Art
To process color image information digitally and acquire an image for reproduction and other purposes, operations such as processing for decomposing an image in a color original into red-green-blue (RGB), reading it and converting it to color space according to L*a*b* system of color representation (L*: lightness, a*b*: hue and saturation) are executed. It is known that generally, man's sensitivity to color difference is extremely high.
As described in No. 3, vol. 14 of "Performance testing of color-difference metrics using a color tolerance dataset" written by D. H. Alman, R. S. Berns, G. D. Synder and W. A. Larsen and published in June, 1989 by COLOR research and application, if color difference .DELTA.E between images to be compared is approximately 5 in an L*a*b* system of color representation, it can be identified regardless of an observer and a status, and if it is not to be recognized, a color difference between images has to be approximately 3. If the target level of the reproducibility of an image is set to a level exceeding the limit of man's recognition of color difference based upon the above fact, a value required for a color image formation apparatus is a very high value equivalent to the color difference of 3 or less.
However, as well-known, each process is instable in conventional type electrophotographic method and it is impossible to meet a high color difference value. This is because the electrophotographic method utilizes a phenomenon caused by static electricity, the image output state of an apparatus itself varies due to a service and environmental condition such as temperature and humidity and the aging and the like of photosensitive material and developer, and the reproducibility of an image fluctuates. In a color image formation apparatus using an electrophotographic method, feedback control is generally used to keep the density of an image suitable.
As a concrete example of feedback control, there is a method of calculating the set value correction amount of an actuator for control by determining an error between density in a density patch and target density,and multiplying it by a feedback gain so as to monitor a state in which density is reproduced as well as an environmental condition in an apparatus using a density patch. For example, in an image formation apparatus disclosed in Japanese Published Unexamined Patent Application No. Hei 1-169467, desired image density is acquired by measuring density in a density patch and controlling the conditions of exposure and developing bias. For a density patch, an unfixed toner image density patch after a developing process or a density patch of a fixed image formed on a transfer medium such as paper after a fixing process is used.
A reason why a toner image density patch is used is that a developed image can be more readily generated or erased than a transfer image or a fixed image created on paper. However, although a toner image density patch is closely related to fixed image density, the effect of variation in a transfer process which is a postprocess cannot be detected using the toner image density patch. In the meantime, a reason why a fixed image density patch is used is that the type of an image is an image itself which a user finally obtains and the quality of an image can be evaluated including the cause of variationin a transfer process and a fixing process.
An image formation apparatus monitoring fixed image density is disclosed in Japanese Published Unexamined Patent Applications No. Sho 62-296669, No. Sho 63-185279, No. Hei 5-199407 and others, and each case utilizes an image reader built in the body of each apparatus.
However, if an image reader is utilized, a user is required to send an image once output to the image reader and read it again so as to detect the image and it is very troublesome for the daily management of image quality. In the case of a color image formation apparatus not provided with an image reader such as a printer, an image cannot be detected in principle.
The applicant of the present invention proposes a color image formation apparatus for enabling monitoring an output image online after a fixing process in Japanese Patent Application No. Hei 7-332373. A sensor for monitoring a color image used in a color image output device in the above application uses the light emitting diodes (LED) of blue (B), green (G), and red (R) corresponding to each monochrome toner of yellow (Y), magenta (M), and cyan (C) as a light source and is constituted so that reflected light from an output image is received by a photodiode.
Generally, the band of the emission spectrum of LED is narrower than one of a spectrum by RGB filter, etc. and it is said to be difficult to divide all colors into a spectrum precisely. However, by limiting the conditions of use of a sensor with an LED light source forming an image output for a monitor as a color patch consisting of monochrome toner such as Y, M, and C and detecting the quantity in which each monochrome toner adheres, that is, the density of each toner, a sensor for a monitor can be provided which is by far more advantageous in cost and size than a conventional type color sensor which divides all colors into a spectrum and identifies each of full colors and is necessary and sufficient in performance. As described above, the reproducibility of the quality of each monochrome toner image can be enhanced by detecting the density of monochrome toner and controlling image formation conditions.
In the meantime, a normal full color image is seldom formed by only individual monchrome toner. Rather an image is formed by overlapping plural types of color toner. However, as transfer efficiency varies depending upon environment in an apparatus such as temperature and humidity, the transfer order of each toner and the like in the above overlapping process, that is, in a transfer process, the transfer efficiency of monochrome toner and that of overlapped color toner, may not coincide. As a result, even if the reproducibility of the quality of each monochrome toner image is excellent, a target image quality cannot be reproduced in a full color image in which plural types of color toner are overlapped simply because of that. Therefore, it is verified that it is difficult to acquire neutral gray, that is, gray balance reproduced by three primary colors of Y, M, and C.
A color image formation apparatus disclosed in Japanese Published Unexamined Patent Applications No. Hei 5-333652, No. Hei 6-30271, No. Hei 6-171154 and others relates to the fluctuation of the above transfer efficiency. In these apparatuses, image quality control in consideration of effect by the fluctuation of transfer efficiency is executed by detecting not only the density of a toner image after development, that is, before transfer, but also the density of a toner image after transfer and calculating the transfer efficiency based upon the ratio between them of the density.
However, a color image formation apparatus which detects the density of a toner image before and after transfer requires at least two toner image density sensors for every color. For example, in the case of a color image formation apparatus provided with an image formation unit every color of yellow (Y), magenta (M), cyan (C), and black (BK) requires maximum eight (four colors x two units) toner image density sensors and there arises a problem in the size and cost. As infrared rays are normally used for the light source of a toner image density sensor, each color toner of Y, M, and C cannot be identified. Therefore, in the case of an image in which plural types of toner is overlapped, it is extremely difficult to identify and detect the density of a monochrome toner image after transfer and precise gray balance cannot be acquired. In Japanese Published Unexamined Patent Application No. Hei 6-186805, a color image formation apparatus which detects the density of each color toner image using a color sensor on the market is disclosed. However, if the density of each color toner image is detected using a color sensor on the market, light emitted from a light source is scattered when the light transmits the unfixedtoner layer of each color for forming an image. Therefore, reflected light provided with a spectrum reflected by a toner layer on the surface of a photoconductive drum fully occpies a spectrum detected by a sensor based upon reflection.
Therefore, it is also extremely difficult to identify the color of toner except the surface layer based upon an unfixed toner image by a sensor such as a color sensor using not infrared rays but light which can be divided into a spectrum and precise gray balance cannot be acquired. Further, it is impossible in principle for a toner image density sensor to detect an image after a fixing process which is an output image that a user finally obtains.